The invention described herein relates generally to apparatus and method for determining the accuracy of machines having servo drives, and more particularly to apparatus and method for determining the accuracy of machine tools, including robots, and those measuring machines having non-disengagable servo drives which cannot be clutched out.
Many measuring machines and robots, and all machine tools have non-disengagable servo drives which cannot be clutched out. Many of these machines, which include lathes and milling machines, are numerically controlled and may be programmed to direct their active element to execute predetermined spatial trajectories. These machines must be tested to very precisely determine the accuracy to which those trajectories can be maintained. There are known methods of doing this. Unfortunately these methods require both elaborate apparatus and a great deal of time. One such method involves what are known as `parametric` tests which can require as much as two weeks to perform. These tests involve equipment such as straight edges, squares, laser interferometers, and gage blocks. Individual measurements are made of those parameters which determine the volumetric accuracy of the machine being tested. These parameters include, among others, straightness and squareness of travel, accuracy of angular motion, and accuracy of linear displacement.
Another method, involving what are known as two and three dimensional ballplates, requires very heavy and costly to construct apparatus which must first be calibrated as a primary standard. This initial calibration process requires a great deal of time and the apparatus can easily be rendered out of tolerance by any improper handling. The apparatus includes large table-top-like platforms into which are bored several holes. Vertical rods of equal or different lengths, each tipped with a spherical ball-like member, are secured in the holes. The relative positions of the centers of the elevated balls are measured with a high precision measuring machine. The evaluation is performed by seeing how accurately the machine being tested can determine these known positions. This method suffers the disadvantage of providing relatively few test positions at only a small number of vertical heights.
Measuring machines not having a servo drive or having a disengagable servo drive which can be clutched out, often termed frictionless coordinate measuring machines, have been tested with what are known as ball bar standards. These standards comprise rigid bars tipped on both ends with spherical ball-like members. This test involves the determination by the machine being evaluated of the center-to-center distance of the ball bar balls. This technique serves primarily as a means of verifying volumetric accuracy, and is not a diagnostic tool in the sense of the more conventional techniques which can also assess straightness and orthogonality.
A relatively new technique for the testing and evaluation of frictionless coordinate measuring machines involves the use of a rigid ball bar standard, substantially as described in the preceding paragraph, each ball of which is held within a magnetic socket knuckle assembly. One knuckle assembly is attached to the measuring machine while the other is free to move about on the spherical surface determined by ball bar length. The machine is evaluated by having it determine, for various orientations, the center-to-center spacing of the ball bar standard balls. This technique is not applicable to machine tools or measuring machines having non-disengagable servo drives which cannot be clutched out.
Thus, at the present time, a major problem facing the machine tool, robot, and measuring machine industries is the inability to quickly, easily and economically determine the accuracy of machine tools, robots, and those measuring machines having non-disengagable servo drives which cannot be clutched out.